Electrical safety device with improved trip mechanism

ABSTRACT

Electrical safety device for controlling the flow of current in a conductor and interrupting the flow in the event of a predetermined current such as an excessive current or ground fault current.

United States Patent Doyle 1 1 Feb. 4, 1975 1 1 ELECTRICAL SAFETY DEVICE WITH IMPROVED TRIP MECHANISM [56] References Cited [75] Inventor: Richard C. Doyle, Benicia, Calif. UNITED STATES PATENTS Assignee: The Rucker Company cnncord 3,217,] I BI'HCIQIII Calif. 3,315,190 1/1957 Mouls ...135/21 3,594,668 7/1971 Clarke et al. 335/38 [22] Filed: Mar. 20, 1974 3,623,150 11/1971 Minns 335/192 335 21) pp, No: 452,956 3,737,821 6/1973 Morton et a1 (1 Related U-S- Application ata Primary Examiner-Harold Broome [60] Division of $61. No. 271,988, July 14, 1972, which Attorney. g or i h H bach, Test,

is a continuation-impart of Ser. No. 141,151, May 7, Albritton & Herbert 1971, abandoned, which is a continuation-in-part of Ser. No. 87,713, Nov. 9, 1970, abandoned. ABSTRACT [521 US. Cl 335/21, 200/D1G. 42, 335/18, Electrical safety device for controlling the flow of cur- 335/166, 335/173 rent in a conductor and interrupting the flow in the [51 Int. Cl. H0111 73/02 event of a predetermined current such as an excessive [58] Field of Search 335/18, 26, 172, 173, 192, current or ground fault current.

335/194, 82, 38,15,16, 21, 25,174, 200, 166; 337/168, 170; ZOO/DlG. 42

i m! ii iii 5 Claims, 17 Drawing Figures Pmmzurw 4% 3.864 649 SHEET u (1F 5 FIG. I I

FIG. 12 04 ELECTRICAL SAFETY DEVICE WITH IMPROVED TRIP MECHANISM CROSS REFERENCE TO RELATED APPLICATIONS This is a division of copending application Ser. No. 271,988, filed July l4. I972, which was a continuationin-part of then pending application Ser. No. l4l,l5l, filed May 7, 1971, now abandoned, which was a continuation-in-part of then pending application Ser. No. 87,7I3, filed Nov. 9, I970, now abandoned.

SUMMARY AND OBJECTS OF THE INVENTION This invention pertains to an electrical safety device for controlling the flow of current in a conductor and interrupting the flow in the event of a predetermined current such as an overcurrent, a short circuit or a ground fault current. The device includes a set of contacts movable between open and closed poaisitons for controlling the flow of current in the conductor, a floating contact arm carrying one of the contacts, a latch member releasably engaging the arm member. and resilient means engaging the arm member between the contact carried thereby and the latch member for urging the contacts together when the latch member is engaging the contact arm. The resilient means also serves to urge the contact arm to pivot about the contacts to provide a rocking movement between the contacts when the latch member is disengaged from the contact arm. A fulcrum member is positioned for engaging the contact arm between the contact carried thereby and the resilient means when the arm has pivoted a predetermined distance about the contacts and thereafter cooperating with the resilient means to separate the contacts.

It is in general an object of the invention to provide a new and improved electrical safety device for controlling the llow of current in a conductor and interrupting the flow in the event of a predetermined condition in the current.

Another object of the invention is to provide an electrical safety device of the above character having an improved trip mechanism.

Additional objects and features of the invention will be apparent from the following description in which the preferred embodiments are set forth in detail in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a block diagram of one embodiment of an electrical safety device incorporating the invention.

FIG. 2 is a front elevational view of one embodiment of an electric receptacle assembly with ground fault protection incorporating the invention.

FIG. 3 is a side elevational view, partially sectioned, of the receptacle assembly illustrated in FIG. 2.

FIG. 4 is a bottom plan view, partially sectioned. of the receptacle assembly illustrated in FIG. 2.

FIG. 5 is a cross-sectional view taken along line 5-5 in FIG. 4.

FIG. 6 is a cross-sectional view taken along line 6-6 in FIG. 3.

FIG. 7-10 are fragmentary cross-sectional views illustrating the operation of the trip mechanism of the receptacle assembly shown in FIGS. 2-6.

FIG. 11 is a front elevational view of a second embodiment of an electric receptacle assembly with ground fault protection incorporating the invention.

FIG. 12 is a cross-sectional view taken along line l2--l2 in FIG. 11.

FIGS. 13-15 are fragmentary cross-sectional views illustrating the operation of the trip mechanism of the receptacle assembly shown in FIGS. 11 and 12.

FIG. 16 is a cross-sectional view taken along line l6l6 in FIG. 13.

FIG. 17 is a wiring diagram of the receptacle assembly of FIGS. 11-16.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the drawing, the invention is illustrated in connection with a single phase. two wire system consisting of an AC. source 15, an ungrounded line conductor L and a ground neutral conductor N. The neutral conductor N is connected to ground at its source end. Although illustrated as a direct connection, the ground return does not have to have a resistance of zero ohms. The invention will operate satisfactorily with a resistance as large as approximately 2000 ohms in the ground return.

The safety device illustrated in FIG. 1 is a receptacle assembly having socket means 16, interrupter contacts 17 connected for controlling the flow of current between the conductors L and N and the socket means 16, ground fault detector means 18, and operating means 19 for opening the contacts 17 when a ground fault is sensed by the detecting means 18. Reset means 21 is provided for closing the contacts 17 after a ground fault has been corrected.

The socket means 16 provides means for delivering electrical energy from the conductors of the distribution system to an electrical appliance such as the load 22. In the preferred embodiment, this socket means is a conventional three prong socket having a line termi nal 23 for connection to the line conductor, a neutral terminal 24 for connection to the neutral terminal, and a ground terminal 26 which is connected to ground. The load 22 typicall includes a line cord or service cord 27 which includes a connecting plug 28 having a plurality of current carrying prongs 29. The socket means 16 is adapted for receiving the plug 28 in such manner that the prongs 29 contact the terminals therein.

In one preferred embodiment, the ground fault detecting means 18 is of the flux storage type which is disclosed in US. Pat. No. 3,6l4,533, issued Oct. 19, 1971. This means includes a differential transformer 31 having a toroidal core 32, single turn primary windings 33 and 34 connected in series with the line and neutral conductors and passing through the toroidal core, and a secondary winding 36 consisting ofa plurality ofturns wound on the core. Under normal conditions, that is in the absence of a ground fault, the currents in the primary windings 33 and 34 are balances, producing a zero net flux in the core 32 and a zero output voltage across the secondary winding 36. Upon the occurence of a ground fault, the currents in the primary windings 33 and 34 are no longer equal. a non-zero net magnetic flux is produced in the core 32, and a fault signal is induced in the secondary winding 36. As is described more fully in the referenced copendin g application, this fault signal is applied to one or more reactor cores where it is stored in the form of a flux signal. The stored flux signal is read out of the reactor cores by means of a pulsating signal and after suitable conditioning. such as amplification. is applied to the operating means l9 for actuating the contacts [7. The portion of the ground fault responsive means other than the differential transformer 31 is represented by the block 37 in PK]. 1. This portion is conveniently constructed in a small package such as an integrated circuit. and if desired. it can include means for protecting against undesired grounding of the neutral conductor at or near the load. as described in US. Pat. No. 3.611.035. issued Oct. 5. l97l. and in US. Pat. No. 3.506.906. issued Apr. l4. l970. Alternatively. the ground fault detecting means 18 can be of the modulating type disclosed in US. Pat. No. 3.597.656. issued Aug. 3. l97l.

As is described more fully hereinafter. the operating means 19 includes a trip coil connected for energization by the ground fault responsive means. together with mechanical linkage for opening the contacts when the trip coil is energized.

Means is provided for testing the operating of the ground fault protector. This means includes a resistor 38 and a switch 39 connected in series between the line and neutral conductors on opposite sides of the differential transformer.

As illustrated in FIGS. 2-6. the receptacle assembly is adapted to be mounted in a standard receptacle wiring box 4! of the duplex type. Boxes ofthis type are on the order of 4 inches long. 2 inches wide and 2 inches deep. and they are commonly used for housing single socket and/or switch assemblies. Box 41 is illustrated as being mounted in a recessed area formed in a building wall 42. A cover plate member 43 is mounted on the outer surface of the wall 42 over the recessed area and provides means for enclosing the box 41. The box 4] is typically fabricated of an electrically conductive metal. and the cover plate is typically fabricated of an insulative material such as plastic.

The receptacle assembly includes a housing 44 which is mounted inside the standard box 41. This housing is generally rectangular in shape and is fabricated of an insulative material such as Bakelite. The remaining components of the receptacle assembly are mounted within the housing 44. This housing is open at the top and is formed to include arc discharge openings 46 in proximity to the electrical contacts which are mounted therein. The housing 44 is mounted to the box 41 by means of a mounting plate 47. This plate is formed to include mounting tabs 48 which engage ear portions 49 on the insulative housing 44. The mounting plate is secured to the metal box by means of screws and the cover plate 43 is attached to the mounting plate by means of screws 52.

A generally planar rectangular base member 56 is mounted within the housing 44 and rests upon ledges 57 which are formed in the side walls of the housing. Conventional socket assemblies 58. 59 are mounted on the upper side of this base member proximate the ends thereof. Each of these socket assemblies includes a line terminal 23. a netural terminal 24, and a ground terminal 26 for receiving the connecting prongs of a plug such as the plug 28. The line terminals of the two socket assemblies are connected together by means of a conductor 61 disposed in a recess in the upper surface of the base member. and the neutral terminals are connected together by means ofa conductor 62 which is disposed in a recess formed in the bottom surface of the base member. The ground terminals are connected together by means of a conductor 63 which is also disposed in a recess formed in the bottom surface of the base member. Thus. as will be apparent to those familiar with the art. the two socket assemblies are connected electrically in parallel.

The differential transformer 31 and remainder of the ground fault responsive means 37 are mounted in the space between the base member 56 and the bottom wall of the insulative housing.

The contacts l7 and operating means I) are mounted on the top side of the base member between the socket assemblies 58 and 59. Two sets of contacts are provided. one for interrupting the flow of current in the line conductor and the other for interrupting the flow of current in the neutral conductor. Each of these sets of contacts includes a fixed upper contact 66 and a movable lower contact 67. An insulative mounting post 68 is mounted to the base member 56 intermediate the two sets of contacts. The upper contacts in the two sets are carried by rigid conductors 69 which are supported at one end by the insulative post 68 and are connected at their other ends to the line and neutral terminals of the socket assemblies. Thus. one of the conductors 69 connects one of the upper terminals 66 to the neutral terminal of the socket assembly 58. and the other conductor 69 connects the other upper contact to the line terminal of the socket assembly 59. Flexible conductive pigtails H are connected to the movable contacts in the two sets, and these pigtails pass through the toroidal core of the differential transformer to form single turn primary windings. The pigtails also pass through openings formed in the bottom wall of the insulative housing and are connected to the line and neutral conductors by conventional connectors. such as wire nuts 72. in the space between the insulative housing and the standard outlet box. A third pigtail 73 is connected to the ground conductor 63 and passes through an opening in the insulative housing for connection to a grounded conductor 74 by means of a wire nut 72. Alternatively. if desired. the flexible pigtails 71 can be terminated at wiring posts mounted on the base member 56, and the connections to the wire nuts can be completed with conventional insulated wire such as 4 stranded copper wire. In this alternative arrangement. the insulated wires. rather than the flexible pig tails. pass through the differential transformer core and the openings in the housing.

The movable contacts 67 are carried by a contact arm member 76 which is fabricated of an insulative material. This member is generally T-shaped, with the contacts being mounted on the upper surface of the cross bar portion. A roller 77 is pivotally mounted in a notch formed in the base portion of this member. As is discussed hereinafter. means is provided for rotating the arm member 76 to move the contacts between open and closed positions.

The operating means includes a supporting framework which is also mounted on the upper side of the base member between the socket assemblies. This framework includes a base portion 81, a vertically extending back wall 82, vertically extending side walls 83 and a front wall 84. A solenoid 86 is mounted in the space between the front. back and side walls. This solenoid includes a trip coil 87 which is adapted for connection to the ground fault responsive means by conductors 88, and it also includes a plunger member 89 movable between advanced and retracted positions in accordance with the energization of the trip coil. The supporting framework can serve as part of the magnetic circuit for operating the solenoid plunger and is preferable fabricated of a good magnetic material such as a mild steel.

An upper pivot rod 91 and a lower pivot rod 92 extend horizontally between the side walls 83. A latch member 93 is pivotally mounted at one end on the upper pivot rod 91 and releasably engages the contact arm roller 77 at its other end. The latch member is linked to the armature member 89 by means of a connecting pin 94. A latch spring 96 is provided for urging the latch forward into engagement with the contact arm roller 77.

A generally U-shaped resilient member 97 is mounted on the lower pivot rod 92 and engages the contact arm member 76 intermediate the contacts 67 and the roller 77. This resilient member urges the lower contacts 67 up against the latch member 93. The post 68 is formed to include a fulcrum portion 98 in proximity to the upper surface of the arm member 76 intermediate the contacts 67 and the resilient member 97.

Means is provided for resetting the operating means and moving the contacts to their closed position following the correction of a ground fault. This means includes a generally cylindrical member 101 which is slidably mounted in a vertically extending bore 102 formed in the post 68. The lower extremity of the member engages the upper surface of the arm member 76 at a location intermediate the resilient means 97 and the lower contacts 67, thus providing means for applying a force to the contact arm generally opposite to the force applied by the resilient means 97. The cylindrical member 101 extends through an opening 103 in the cover plate 43 and through an opening 104 in the mounting plate 47. This member is movable between extended and retracted positions, and a light spring 106 is provided for holding it in its retracted position. A colored band 107 is provided on the member in such position that it is visible only when the member is in its extended position. As will appear hereinafter, the operation of the reset means is such that the contacts cannot be manually closed by it while a ground fault is present. The resistor 38 is mounted in a vertical bore in the post 68. In one terminal of this resistor is connected to the neutral pigtail on the source side of the differential transformer. In one presently preferred embodiment, this connection is made by means of an insulated wire which passes through the differential transformer core and then is connected to the neutral wiring post on the base member. The other resistor lead 111 is cut off close to the resistor and disposed in proximity to a resilient contact member 112 which is connected to the upper contact 66 connected to the line terminals of the sockets. An insulative test button 113 engates the contact member 112 and passes through an opening 116 in the frame member 47. This button extends part of the way through an opening 114 in the cover plate 43 and is accessible from outside the cover plate by means ofa small or pointed instrument. Being recessed, however, it is not likely to be accidentally depressed.

Operation and use of the electrical safety device can now be described briefly. Initially. let is be assumed that an electrical appliance has been connected to one of the sockets and that the system is operating normally. that there is no ground fault. In this condition.

the trip coil 87 is deenergized. and the resilient member 96 urges the latch member 93 forward into engagement with the roller 77 of the contact arm member 76. The resilient member 97 urges the contact arm member upward. holding the contacts in their closed position. The reset member 101 is urged downward against the contact arm member by the spring 106. and it remains in its retracted position with the colored band 107 concealed by the cover plate 43.

Upon the occurrence ofa ground fault. or when the test button 113 is depressed, as illustrated in FIG. 7. the trip coil 87 is energized. This moves the solenoid plunger member 89 to its retracted position. drawing the latch member 93 out of engagement with the contact arm member roller 77. In this condition. the contacts 66 and 67 function as a fulcrum. and the resilient member 97 moves the roller end of the contact arm in an upward direction, producing a rocking action at the contacts. This rocking action has been found to be very desirable since it tends to break any contact welds which may have formed between the contacts. The rocking action continues until the contact arm engages the fulcrum portion 98 of the post 68. At this point, the center of rotation shifts from the contacts to the fulcrum portion 98, and the contacts are separated. As the contact arm rotates, it pushes the reset member 101 upward to its extended position. exposing the band of color 107 to provide a visible indication of the occurrence of the ground fault. The rotational movement of the contact arm continues until its contact end engages the upper surface of the base member 56. The assembly is shown in its fully tripped position in H0. 8.

In order to reset the operating means and close the contacts, the reset member 101 is manually depressed. This exerts a force on the contact arm member opposite to the force exerted by the resilient member 97. The contact arm now pivots around the contact end which is engaging the upper surface of the base mem' ber 56, and the roller end moves downward. The resilient member 96 moves the latch member 93 back into position for engaging the roller 77. As long as the reset member 101 is depressed, the contacts will remain open, as is best seen in FIG. 9. When the reset means is released, the contact arm 76 moves upward until the roller engages the latch member 93. Thereafter, the contact arm rotates about the roller, with the contact ends thereof moving upward until the contacts are in their closed position. Thereafter, the device will again be in the normal operating conditions illustrated in H0. 6.

It is to be noted that the protective operation of the device cannot be defeated by the reset means. As long as the reset member 101 is depressed, the contacts will remain open. When the trip coil 87 is connected to remain energized throughout the ground fault, the unit cannot be reset until the ground fault is removed since the solenoid armature 89 will be in its retracted position, preventing the latch member 93 from engaging the roller 77. If the trip coil is deenergized by the ground fault, the device will trip immediately if an attempt is made to reset it during the continuance of the ground fault.

The receptacle assembly shown in FIGS. 11-17 includes a housing assembly 121 which is preferably fabricated of an insulative material. A cover plate 122 is mounted on the housing assembly by means of a mounting plate 123. and the entire assembly is mounted in a standard duplex box I24. The housing assembly includes a raised portion I2Ia which extends through a generally rectangular opening I220 in the cover plate. A pair of three conductor outlets I26 and I27. a reset button 128. and a test button I29 are all mounted in the raised portion of a housing assembly and accessible from the outer side of the cover plate. The line, neutral and ground terminals of outlets I26 and 127 are connected electrically and parallel by conductors I3l-I33.

The flow of current to conductors I3] and I32 and. hence. to outlets I26 and I27 is controlled by a contact assembly I34. This assembly includes two sets of contacts. one for the line conductor and the other for the neutral conductor. Each set includes a fixed contact I36 and a movable contact 137. with the fixed contacts being connected to the line and neutral terminals of the outlets. The movable contacts are mounted on a contact arm I38 and connected to flexible pigtails 139 which are connected to the protected line and neutral conductors. Conductor 133 is connected to ground by a third pigtail 141.

Contact arm I38 is fabricated of an insulative material. and it is generally T-shaped. Contacts 137 are mounted on the cross bar portion of the arm. and the other end of the arm is releasably engaged by a latch 142 which forms a part of a clapper type solenoid assembly 143. This assembly includes a trip coil 144 and the latch or clapper 142. The coil is mounted on a generally U-shaped bracket I46, and the clapper is pivotally mounted on a pivot rod I47 which extends between the arms of the U-shaped bracket. A spring I48, carried by rod 147, engages the central portion of arm I38 and urges the arm in an upward direction, as viewed in FIG. I2. A latch spring I49 urges latch or clapper I42 toward the latching position shown in FIG. I2.

The operation of solenoid assembly I43 is controlled by a ground fault sensing module I5I which is mounted on the lower portion of housing assembly I2I. In the preferred embodiment, the module contains the circuit shown in FIG. I9 which. as disclosed in copending application Ser. No. MI .297, filed May 7. I971. can be constructed in a compact modular form. The module includes a window 152 through which the line and neutral pigtails 139 pass to form the single turn primary windings of a differential transformer. One end of solenoid coil 144 is connected to the module by a lead I53, and the other end of the coil is connected to a switch contact 154 which is normally engaged by a movable contact 156. A second movable contact I57 engages contact 156 when test button I29 is depressed. One lead I58a of a resistor I58 is connected to contact 157, and the other lead l58b of this resistor is connected to the line conductor through fixed interrupter contact I36.

Reset button I28 is movable between extended and retracted positions, and a spring I61 urges the button toward its retracted position. as illustrated in FIG. 12. The button is provided with an indicator band 128a which can be seen from the outer side of the cover plate only when the button is in its extended position. When the button is in its retracted position, the lower portion of the button extends somewhat below a shoulder I62 formed by the housing assembly.

Operation and use of the receptacle assembly shown in FIGS. II17 can now be described. Initially. let it be assumed that an electrical appliance has been connected to one of the outlets and that the system is operating normally, that is, there is no ground fault. In this condition. solenoid coil I44 is deenergized. and latch spring 149 urges latch or clapper I42 into engagement with the end of contact arm 138. Spring I48 urges the contact arm upward. holding the contacts in their closed position. Reset button I28 is urged downward against the contact arm by spring I6I. and it remains in its retracted position with indicator band 128a concealed by cover plate 122.

If test button I29 is depressed. as illustrated in FIG. I3, contact I57 engages contact I56. connecting resistor 158 between the line and neutral conductors on opposite sides of ground fault sensing module I5l. creating an imbalance in the currents in the protected conductors. The module energizes solenoid 153 in response to this imbalance. as it would in the event of a ground fault. and latch 142 is drawn out of engagement with contact arm 138. In this condition. contacts 136 and I37 function as a fulcrum. and spring 148 moves the free end of the contact arm in an upward direction. producing a rocking action at the contacts. This action continues until the contact arm engages the shoulder portion 162 of the housing assembly. At this point, the center of rotation shifts from the contacts to the shoulder portion, and the contacts are separated, as illustrated in FIG. 4. As the free end of the contact arm moves upward. it engages contact I56. moving this contact out of engagement with contact I54 to deenergize solenoid coil I44. It will be noted that interrupter contacts 136 and 137 separate before contact I56 is moved, thereby assuring interruption of the current in the protected conductor before the solenoid coil is deenergized. When the coil is deenergized, spring 149 urges latch I42 to back into position for engaging the free end of the contact arm when the arm is reset to close the contacts. With the contacts open, contact arm I38 moves reset button I28 to its extended position, exposing indicator band 128a.

After the ground fault has been corrected, the assembly can be reset by depressing reset button I28, as indicated in FIG. I5. Depressing the button moves the free end of the contact arm downward into latching engagement with latch I42. When the button is released, spring 148 will move the other end of the contact arm upwardly. closing contacts I36 and 137. It will be that when the reset button is first depressed, the free end of contact arm I38 releases contact 156 which then moves back into contact with contact 154, completing the circuit to solenoid coil 144. Thus, if someone attempts to reset the assembly before a ground fault is corrected, the unit will trip again immediately upon the closing of the contacts 136 and I37.

It is apparent from the foregoing that a new and improved electrical safety device has been provided. While only the presently preferred embodiments have been described. as will be apparent to those familiar with the art. certain changes and modifications can be made without departing from the scope of the invention as defined by the following claims.

I claim:

I. In an electrical safety device for controlling the flow of current in a conductor and interrupting the flow in the event of a predetermined current: first and second contacts for making and breaking a circuit with the conductor, a floating contact arm having the first contact mounted thereon and movable relative to the second contact, a latch member releasably engaging the contact arm at a point spaced from the first contact. resilient means engaging the contact arm between the first contact and the latch member for urging contacts together when the latch member is engaging the contact arm. said resilient means serving to pivot the arm about the contacts to provide rocking movement between the contacts when the latch member is disengaged from the arm. means for disengaging the latch member from the contact arm in response to the mode termined current, and a fulcrum member positioned for engaging the contact arm between the first contact and the resilient means when the arm has pivoted a predetermined distance about the contacts and thereafter cooperating with the resilient means to separate the contacts.

2. The electrical safety device of claim I wherein the means for disengaging the latch member includes an operating coil.

3. The electrical safety device of claim 1 further including a manually operable reset member engaging the contact arm for exerting a force on said arm generally opposite to the force of the resilient means to return the arm to a position in which said arm can be engaged by the latch member.

4. The electrical safety device of claim 3 further including a resilient member urging the latch member into position for engaging thc contact arm.

5. The electrical safety device of claim 3 wherein the means for disengaging the latch member is adapted for holding the latch member out of engagement with the contact arm as long as the condition producing the predetermined current is present.

* li 1' 1 i 

1. In an electrical safety device for controlling the flow of current in a conductor and interrupting the flow in the event of a predetermined current: first and second contacts for making and breaking a circuit with the conductor, a floating contact arm having the first contact mounted thereon and movable relative to the second contact, a latch member releasably engaging the contact arm at a point spaced from the first contact, resilient means engaging the contact arm between the first contact and the latch member for urging contacts together when the latch member is engaging the contact arm, said resilient means serving to pivot the arm about the contacts to provide rocking movement between the contacts when the latch member is disengaged from the arm, means for disengaging the latch member from the contact arm in response to the predetermined current, and a fulcrum member positioned for engaging the contact arm between the first contact and the resilient means when the arm has pivoted a predetermined distance about the contacts and thereafter cooperating with the resilient means to separate the contacts.
 2. The electrical safety device of claim 1 wherein the means for disengaging the latch member includes an operating coil.
 3. The electrical safety device of claim 1 further including a manually operable reset member engaging the contact arm for exerting a force on said arm generally opposite to the force of the resilient means to return the arm to a position in which said arm can be engaged by the latch member.
 4. The electrical safety device of claim 3 further including a resilient member urging the latch member into position for engaging the contact arm.
 5. The electrical safety device of claim 3 wherein the means for disengaging the latch member is adapted for holding the latch member out of engagement with the contact arm as long as the condition producing the predetermined current is present. 